1. Field of the Invention
The present invention relates to a fuel cell, and more particularly to a matrix of a phosphoric acid fuel cell and a method for making it.
2. Description of the Prior Art
As is generally known, a matrix of a phosphoric acid fuel cell is sandwiched between an air electrode to which an oxidizing agent gas such as air is supplied and a fuel electrode to which a reducing agent gas such as hydrogen gas is supplied. The matrix has a function of retaining phosphoric acid as an electrolyte, separating the oxidizing agent gas from the reducing agent gas so as not to mix and contact each other and intercepting the electric conduction by electrons between the electrodes. Hence, it is required that the matrix have sufficient corrosion resistance against phosphoric acid high electric insulation characteristics in addition to a high porosity for retaining phosphoric acid and excellent strength to resist the differential pressure.
Under these circumstances, silicon carbide has been generally used as a main material of the matrix in a conventional phosphoric acid fuel cell and a fine powder of silicon carbide has been bound with a binder such as a fluorine-containing resin, and then made into the formula sheet to construct the matrix.
In order to improve the output capacity of the phosphoric acid fuel cell, it is proposed to make the thickness of the matrix as thin as possible to reduce the resistance overvoltage. However, the physical property of silicon carbide such that is semiconductor-like, and the electric resistance of silicon carbide is low in comparison with that of an insulating material. Therefore, when the thickness of the matrix is made thin, electric conduction is caused in the fuel cell by the movement of electrons between the electrodes through the matrix. As a result, the output of the fuel cell reduced.
Silicon nitride is also used as a matrix material as disclosed, for example, in Japanese Laid-open Patent Application No. 165,264/1983.
Since silicon nitride is an insulating material, it does not have the problems discussed above for the case of silicon carbide. However, silicon nitride is expensive when compared with silicon carbide and is not yet practical for use in a fuel cell.